The present invention relates to a semiconductor wafer for providing a plurality of semiconductor chips therein through an electron-beam lithography, and more particularly to a registration mark formed on the semiconductor wafer for the electron-beam lithography.
In the electron-beam lithography, the semiconductor wafer is selectively irradiated with an electron beam. The registration marks are provided on the semiconductor wafer. Before conducting the selective irradiation of the electron beam on a resist film provided on the semiconductor wafer to form a necessary pattern of each semiconductor chip, the electron beam is scanned across the registration mark to estimate the accurate location of the semiconductor wafer or the semiconductor chip. Conventionally, the registration mark has a cross shape of protrusion or depression. In general, the registration mark can be classified into a wafer registration mark having a large size and provided in four peripheral portions of the semiconductor wafer, respectively, and a chip registration mark having a smaller size than that of the wafer registration mark and provided in four corner portions of each semiconductor chip, respectively. In the alignment procedure, the electron beam is scanned in one direction (say, X-direction on an X-Y plane) to cross a predetermined section of the registration mark, and backscattered high energy electrons are detected by a backscattered electron detector located above the semiconductor wafer. Whenever the scanned electron beam passes the side edges of the predetermined section of the registration mark, a large amount of the backscattered electrons are detected by the detector which thereby outputs peak signals. The position of the registration mark, that is, of the wafer or the chip, in the X-direction can be estimated by the peak signals, and the result is sent to a computer installed in the electron-beam lithography apparatus. In the same manner the position in the Y-direction is estimated. In accordance with the estimated results, the computer controls a X-Y stage control system or an electron beam control system to form the designed pattern on the semiconductor chip.
The output waveform of the backscattered electron detector, however, involves inevitably many noises having relatively high level at random positions. Therefore, it is sometimes difficult to detect the peak signals and to determine the position of the side edges of the registration mark. To solve this problem, in the prior art, the electron beam is repeatedly scanned on the same predetermined section of the registration mark, for example, 10 to 30 times and the output waveforms of the detector which includes both the peak signals and the noises are added. Since peak signals are produced at the fixed positions in every waveforms and the noises are produced at random positions, the added waveform has a high S/N (peak signal to noise) ratio and a precise alignment can be realized. However, when the electron beam is scanned many times, a large amount of electrons are accumulated in the resist film along the passage of electron beam scanning. Therefore, the resist film is subjected to a baked phenomenon, that is, the resist film is tightly fixed to a material such as silicon, silicon oxide, etc. forming the registration mark so that it is hardly removed by a general resist removing solution such as OMR 502 (product of Tokyo Ohka Co., Ltd.), 1112A (product of Shipley Co., Ltd.), etc. If an electron beam having a rectangular plan shape of 12.5 .mu.m.times.1 .mu.m and electrical density of 0.4 A (ampere)/cm.sup.2, for example, is scanned 30 times, charges of about 3000 .mu.c (micro coulomb)/cm.sup.2 is dosed in the resist film. It is to be noted that a resist film used in the electron-beam lithography is subjected to the baked phenomenon mentioned above when the dosage becomes 300 to 1000 .mu.c/cm.sup.2 or more. To remove the baked resist film, hot sulfuric acid (H.sub.2 SO.sub.4) or hot nitric acid HNO.sub.3) must be used. However, in this case, an aluminum film formed in a previous process step would be corroded. Altenatively, oxygen plasma can be used to make dry etching of the baked resist film, but suffered with unfavorable influence to be induced in the semiconductor substrate.
Further, in the prior art, the registration mark has a cross shape in the plan view consisting of one rectangular portion extending in Y-direction and one rectangular portion extending in X-direction and crossing the former portion, and when the alignment in X-direction is conducted, the only one rectangular portion extending in Y-direction is used as the registration mark. Therefore, the registration mark is mistakable with a region of an element pattern having a similar shape with the elongated portion of the registration mark and positioned near the mark. To avoid the erroneous estimation, the difference in height between the surface of the registration mark and the surface of the part near the mark must be 1.0 .mu.m or more. Therefore, in general, the registration mark must be formed at a first process step among a plurality of process steps for forming the semiconductor device, and the mark must be used till the final process step in the wafer state. However, during manufacturing the device, requirements sometimes occur that a pattern formed in one process step must be precisely aligned with another pattern formed through just previous process step. In this case, it is favorable that a new registration mark is formed in the previous process step with the another pattern. However, if the previous process step is of patterning a film of polycrystalline silicon or insulating material having the thickness of 0.2 to 0.5 .mu.m, the registration mark having the thickness of 1.0 .mu.m or more cannot be formed in the previous process step.